docs: def security(policy, app_secret):
      """
      Creates a valid signature and policy based on provided app secret and
      parameters
      ```python
      from filestack import Client, security
  
      # a policy requires at least an expiry
      policy = {'expiry': 56589012, 'call': ['read', 'store', 'pick']}
      sec = security(policy, 'APP_SECRET')
  
      client =  Client('API_KEY', security=sec)
      ```
      """
      validate(policy)
      policy_enc = base64.urlsafe_b64encode(json.dumps(policy).encode('utf-8'))
  
      signature = hmac.new(app_secret.encode('utf-8'),
                           policy_enc,
                           hashlib.sha256).hexdigest()
  
      return {'policy': policy_enc, 'signature': signature}